1. Field of the Invention
This invention relates generally to multiple beam optical systems and more particularly to a system for measuring the individual beam powers of overlapping beams.
2. Description of the Prior Art
Magneto-optic recording provides for erasable storage of data. A write laser beam is focussed onto a spot on the medium and heats the magneto-optic material to a temperature at which the magnetization of the magnetic domains of the medium may be changed. A magnetic field is applied in one of two directions to orient the magnetic domains of the spot in either an upward or downward direction.
The disk is read by focussing a polarized read laser beam onto the magneto-optic material. The read laser beam has a lower power level than the write laser beam. The Kerr effect causes the reflected beam's plane of polarization to be rotated either clockwise or counter-clockwise depending on whether the spot has an upward or downward magnetic orientation. The difference in the rotation is detected and represents the recorded data.
The typical system uses one laser with variable power to do both writing and reading. In order to verify the data recorded, the disk must be rotated almost three times for each track recorded. One and one half revolutions are required on average to seek and write the track and a second revolution is required to read and verify the track just written.
In order to speed up the recording process, direct read after write (DRAW) systems have been proposed. These systems comprise two lasers; one laser (the read/write laser) to write a track and a second laser (the DRAW laser) to read the track directly after it has been written. Thus, the DRAW system requires only one and one half revolutions on average to write and verify a track on the disk. The read/write laser alone is used to read the disk when no writing function is being performed.
A problem with DRAW systems has been that the addition of the second laser beam greatly complicates the construction of the optical channel. The beams must be spaced close together in order to focus on the same track at the same time and to prevent aberration and truncation (beam obstruction) effects. However, this close spacing makes separation and power monitoring of each of the beams difficult.
Power monitoring schemes for lasers are shown in U.S. Pat. No. 4,877,311 issued Oct. 31, 1989 to Shernoff; U.S. Pat. No. 4,733,067 issued Mar. 22, 1988 to Oinoue, et al.; U.S. Pat. No. 4,660,983 issued Apr. 28, 1987 to Yamamoto, et al.; U.S. Pat. No. 4,297,653 issued Oct. 27, 1981 to Scifres, et al.; Japanese patent application 01-160073 published Jun. 22, 1989 by Kaneko, et al.; Japanese patent application 02-79483 published Mar. 20, 1990 by Shinoda, et al.; Research Disclosure 313107 published Apr. 20, 1990; Research Disclosure 313101 published May 10, 1990; and Research Disclosure 290086 published Jun. 10, 1988.
What is needed is a power monitoring scheme for measuring individual beam powers in two overlapping beams.